The present invention is directed to a long-nip press for a paper machine or cardboard machine, and to a method for dewatering a fibrous web, such as a paper web, passing through such a long-nip press.
More specifically, the present invention is directed to a long-nip press for a paper machine or a cardboard machine, in which the press zone is formed between a counter-roll and a press-shoe arrangement, or between a counter-shoe and the press-shoe arrangement. The web of fibre such as paper passes through this press zone between two press belts, or between a felt and a single press belt. The press-shoe arrangement is located inside the loop of the press belt, with the press including means for hydrodynamically lubricating the press belt by feeding lubricant onto an inside face of this press belt before the press zone. The press also includes means for recovering the lubricant, by collecting as much of the lubricant as possible after the press zone.
It is commonly known in the prior art to remove water out of fibrous webs, such as from paper or cardboard webs, by passing such a web through a press nip formed by two rolls disposed opposite one another. As known in the prior art, one or two press fabrics are passed through such dewatering nips, these press fabrics carrying water removed out of the fibrous web, and also acting to convey the fibrous web forwardly.
As the production rate of such paper machines increases, the dewatering performed by means of such nip pressing has become an obstacle limiting increase of speed of paper production. This is due to the fact that the press nip is formed by a pair of rolls having a short pressing area, so that the residence time of a running fibrous web in these nips is very short at very high speeds of operation. However, due to the flow resistance of the fibrous structure of the web itself, water requires a minimum amount of time for escaping out of the web into the hollow face of a press roll or into a press fabric, in a dewatering operation.
Several press nips have been used for dewatering as disclosed in the prior art, examples of which include the so-called "Sym-Press" press section, or several separate, individual, successive press nips. However, the use of successive press nips requires a relatively large area for operation, especially if separate, individual press nips are used one right after the other. Compact construction of press sections however, such as the "Sym-Press" press section, cause difficulty in obtaining optimal positioning of the various components, as well as creating difficulties in the operation of the press itself, such as in the removal of paper broke. In nip presses, suction rolls are commonly used which are relatively expensive components and which consume a tremendous amount of suction energy and cause noise. In suction rolls, a perforated mantle must be used, which causes problems with the mechanical strength of such suction rolls.
If an attempt is made to increase dewatering output in nip presses by increasing the nip pressure, a certain limiting line pressure value is reached, beyond which any increase in the line pressure is no longer helpful because the structure of the fibrous web and of the press fabrics can no longer withstand the increased compression pressure.
Attempts have been made to lengthen the area of roll nips contacted by the web to be dewatered, by using rolls of larger diameter and by using soft press fabric, but even with utilization of these features, a limit in terms of feasible economic application is soon reached.
In order to solve the problems noted above in addition to other problems, so-called long-nip presses have been developed in recent years. Such types of presses are disclosed, for example, in U.S. Pat. Nos. 3,808,092; 3,808,096; 3,840,429; 3,970,515; 4,201,624; and 4,229,253, as well as in GB Pat. Appln. No. 2,057,027.
Further prior art press constructions are disclosed in FI Pat. Appln. No. 3,554/72 and in U.S. Pat. No. 3,783,097, both assigned to Beloit Corporation, U.S.A. In the former document, a press construction for a paper machine is disclosed for pressing water out of a paper web by the use of flexible belts, and by achieving a long press zone by tensioning these flexible belts. However, a drawback of this particular long-nip press is that mechanical strength of the press felts and of the concomitant guide rolls, imposes a restriction on the generation of sufficiently high compression, needed to obtain efficient dewatering.
In U.S. Pat. No. 3,783,097, a long-nip press is disclosed in which several subsequent pressure shoes are utilized, the pressure shoes being pressed towards an opposing belt and press roll. A drawback of the construction disclosed in this reference is that the friction between the pressure shoes and the opposing belt causes a tremendous consumption of energy, with the belt and the press shoes being subjected to intensive wear-and-tear due to the rubbing.
In U.S. Pat. No. 3,840,429, a long-nip press is described in which the web to be pressed linearly runs between two felts through a press zone formed by two opposing press shoes, and generated by means of a pressure medium. Moreover, bands are disposed inside the loops of the felts, to define the press zones and transmit the pressure of the medium to the web. However, problems of sealing of the press zone occur in this particular type of long-nip press, with the patent not suggesting any solution.
Another drawback of this construction is that the web is immediately subjected to full and necessarily relatively high compression pressure. However, since a web has a low dry-solid content, this web will not withstand an initial pressure of compression greater than a certain limit, without breaking. Thus, the compression pressure in a long-nip press of U.S. Pat. No. 3,840,429, must be kept relatively low. In view of the fibrous structure of the running web, it is disadvantageous to immediately use suddenly-increasing, high compression pressures as the initial pressure of compression in a press nip.